1. Field of the Invention
The present invention relates to the preservation of eye tissue in a defined serumfree medical solution, and more particularly, relates to the preservation and enhancement of human corneal tissue, specified as the time between removal from the donor and transplantation.
2. Description of the Prior Art
Keratoplasty, or the transplantation of the cornea, has been effective in providing visual rehabilitation to many who suffer from corneal disorders. This procedure has been severely hampered by the universally inconsistent availability of donor tissue. The use of 4.degree. C. corneal storage medium containing chondroitin sulfate has positively impacted the availability of quality donor tissue. In the United States 95% of all corneas transplanted are stored in a 4.degree. C. chondroitin sulfate containing medium for up to seven days. After 96 hours of preservation the cornea is attended by epithelial decomposition and loss of corneal clarity, as demonstrated by increased swelling of the corneal stroma. The stromal edema is attributed to both the decreased maintenance of the barrier pump function of the corneal endothelium and barrier function of the corneal epithelium.
An alternative to 4.degree. C. corneal storage is the use of organ culture. In this method of corneal preservation, the cornea is maintained at higher temperatures (31.degree. C.-37.degree. C.) allowing greater metabolic activity of the cornea. The use of organ cultured corneas is mainly supported in Europe. The organ culture system utilizes fetal bovine as a major medium component. Mounting concerns over TSEs (Transmissble Spongiform Encephalopathies) stemming from Bovine Spongiform Encephalophy (BSE) outbreaks, have focused much emphasis on animal derived products and their use in corneal preservation. The replacement of serum components in corneal preservation is a formidable challenge, based on over 350 known chemical components found in serum.
The elevated temperature (31.degree. C.-37.degree. C.) of the organ culture technique increases the metabolic rate of the cornea as compared to corneas stored at 4.degree. C. The corneal storage medium must provide an environment similar to the in vivo situation. A serumfree corneal preservation medium must be completely defined as to supplement the components normally found in serum. A critical evaluation of physiologic parameters such as ionic and amino acid composition, bicarbonate equilibrium, available energy sources, dissolved oxygen levels, nutrient cell supplements, coenzymes and enzyme supplements, nucleotide precursors, hormonal supplements, trace minerals, trace elements, growth factors, osmolality and pH should be observed with respect to each preservation medium. Parameters for extended serumfree organ culture preservation should be defined as to the reversibility of cell damage incurred during storage.
Adult corneal endothelium have a limited regenerative capacity and mitotic figures have been rarely observed in vivo; human corneal endothelium in vivo normally responds to trauma by sliding into the wounded area by cell migration. However, in vivo endothelial cell mitosis has been demonstrated in rabbit, bovine and human endothelium. Autoradiographic thymidine uptake studies after cryowounding or mechanical wounding of corneas in vitro has demonstrated existence of mitotic figures in the endothelial monolayer. These studies were all conducted in the presence of serum. Surgical trauma and disease can accelerate the loss of endothelial cells and further compromise the cornea. Thus, the long term preservation and enhancement of the corneal endothelium is a very important aspect of eye bank storage of eye tissue.
An overview of the issues surrounding the storage and handling of corneal tissue is found in Corneal Surgery, chapters 1-4, pages 1-128 edited by Federick S. Brightbill, M.D., published by C. V. Mosby Company, St. Louis, Mo., 1986. A variety of storage media and techniques have been proposed, and current research continues to be directed towards maintaining and actually enhancing the quality of the donor tissues, and increasing the duration of storage of corneal tissues, as defined as the time between excision from a donor and transplantation. Currently, there are no defined serumfree media used in organ culture techniques at 31.degree. C.-38.degree. C.
Accordingly, the present inventions directed towards materials and methods of enhancing ocular tissues, especially corneal tissues, during storage prior to transplantation. One aspect of the invention provides for the enhancement of corneal tissue viability by providing a completely defined serumfree medium that maintains normal physiologic metabolism, and maintains corneal tissue equal to medium that contains serum.